Adjustable speed drives can be used to control the speed at which a machine operates, and commonly incorporate converters having isolation transformers. Power to these transformers is often supplied through a transformer controller having a series of components, such as circuit breakers or switches, contactors and fuses. A current limiting reactor may also be included in the circuit to limit the DC bus capacitor charging current, preventing damage to the capacitors and reducing utility line power disturbances. A conventional approach to limiting DC bus capacitor charging current is discussed below with reference to FIGS. 1 and 2.
FIG. 1 illustrates a conventional drive isolation system 100. The drive isolation system 100 generally couples a three-phase power supply 101 to an end load, such as a motor 102, and includes intermediate circuitry to help provide a desired set of voltages and waveforms to ultimately drive the motor 102. The intermediate circuitry may include a transformer 103 and various components coupled to different secondary windings of the transformer, such as fuses 104, rectifiers 105, DC bus capacitors 106 and inverters 107.
On the input side of the transformer 103, the system may include a transformer controller 108, which is shown in greater detail in FIG. 2. As shown in FIG. 2, the transformer controller 108 includes a manually operated isolation switch 202, fuse 204, main contactor (an electrically-controlled type of switch) 206, precharge reactor 208 and precharge contactor 210. The transformer controller 108 may be controlled by a logic controller 212, such as a programmable device, processor, or control logic circuitry.
During startup operation, or a pre-charge state, the controller 212 causes main contactor 206 to close, while precharge contactor 210 is open. The precharge reactor 208 helps to limit the capacitor charging current that can occur during startup. When the DC bus capacitors (106 in FIG. 1) reach a charged state, the precharge contactor 210 is closed to short out the precharge reactor 208 and remove it from the circuit. After the DC bus capacitors 106 are charged, pre-charge state ends, and the system 100 may enter a post-charge state for normal operations.
One problem with conventional transformer controllers, such as transformer controller 108, is that the kilovolt-ampere (kVA) rating of the components within the transformer controller 108 (e.g., main contactor 206, fuse 204, etc.) must be the same or larger than the kVA rating of the transformer 103. This is particularly disadvantageous when the transformer 106 has a high kVA capacity, because higher rated components are more expensive. There is always a need to lower costs, and a lower-cost transformer controller 108 would be advantageous.